Semiconductor devices are often used in computer and other electronic systems. Older systems often used one basic power-supply voltage, such as 5.0 volts. More recently, lower power-supply voltages such as 3.3 volts have become necessary as semiconductor geometries shrink and applications require lower power drain. As device geometries continue to shrink, lower power-supply voltages are used, such as 2.0 or 1.8 volts.
Many systems use semiconductor integrated circuits (ICs) or chips with several different power-supply voltages, such as 5.0, 3.3, and 1.8 volts. Signals output from a 5.0-volt chip must be reduced in voltage to be input to a 3.3 or 1.8-volt chip. Such voltage translation of output signals is necessary to avoid reading an incorrect logic state, power drain, or latch-up. Furthermore, a 5-volt signal could damage the input thin oxide of a 3.3-volt chip.
Bus switch transistors are often used to connect signals from different chips. Bus switches are semiconductor integrated circuits (IC's) that use metal-oxide semiconductor (MOS) transistors to make or break the connection. Several switches may be combined on a single silicon die. More background on bus switches can be found in "Parallel Micro-Relay Bus Switch for Computer Network Communication with Reduced Crosstalk and Low On-Resistance using Charge Pumps", assigned to Pericom Semiconductor and Hewlett-Packard Company, U.S. Pat. No. 5,808,502. Also see "Bus Switch Having Both P- and N-Channel Transistors for Constant Impedance Using Isolation Circuit for Live-Insertion when Powered Down", U.S. Ser. No. 09/004,929, assigned to Pericom Semiconductor.
FIG. 1A shows a prior-art bus switch device. N-channel transistor 10 connects an input applied to its drain to an output connected to its source. An enable signal is applied to the gate of n-channel transistor 10. This enable signal turns on n-channel transistor 10, connecting the drain to the source. When a low voltage is applied to the enable signal, n-channel transistor 10 turns off, disconnecting the drain and source. This isolates the input from the output.
When the enable or gate voltage is much higher than the source and drain voltages, the output voltage approaches the input voltage so that there is little or no voltage drop across the transistor. However, reduced power-supply voltages make such a gate over-voltage difficult. FIG. 1B shows a bus-switch biased with a 3.3-volt power supply. When the input voltage matches the gate voltage of 3.3 volt, the output (source) voltage is lower than the input voltage by a transistor threshold, Vt. Thus the input voltage of 3.3 volt is reduced by the transistor threshold of about 0.8 volt to produce the output voltage of 2.5 volts. The nominal transistor threshold of 0.7 volt is raised by the body effect when the source is above ground.
Such a voltage drop may be desirable when a 3.3-volt chip is driving a 2.5 volt chip. The bus-switch transistor then acts as a voltage translator, producing a 2.5-volt high signal from a 3.3-volt high signal.
FIG. 1C is an example of voltage translation of a 3.3-volt signal to a 1.8-volt chip. In this example, a 3.3-volt chip sends a signal to a chip with a 1.8-volt power supply. The 3.3-volt input signal must be reduced in voltage to 1.8 volt. Since the maximum output voltage Vout is a threshold below the gate voltage, Vout=Vgate-Vt, or 1.8=Vgate-0.8. Thus the gate voltage Vgate must be 2.6 volts to drop the 3.3-volt input signal to a 1.8-volt output signal.
Generating a 2.6-volt gate voltage is problematic. A voltage divider may be used, but then an additional power drain occurs, and the gate voltage may vary with temperature, process, and supply-voltage variations. Directly inputting a 2.6-volt signal requires an additional output from the system's or PC's power supply, which is undesirable.
Fixed Diode and Transistor Voltage Drops
Voltage drops equal to the transistor threshold voltage may easily be generated, but often the desired voltage is not exactly one threshold below the power-supply voltage. Since the threshold voltage varies with internal voltage due to the body effect, the generated voltage can vary. Temperature, process, and supply-voltage variations can also occur.
Diodes can also be used to reduce voltages. These diodes can be internal or external to the voltage-translator chip. See U.S. Pat. No. 5,751,168 by Speed III et al., and as signed to Texas Instruments. FIG. 2 is a prior-art voltage translator. Diodes 82, 84 are diodes each producing a fixed 0.4-volt drop. Thus the internal power supply 80 is 0.8 volts less than the Vcc power supply. An enable signal ENA is inverted by transistors 86, 88 and again by transistors 92, 94 to drive the gate of voltage-translator transistor 90.
The high gate voltage Vgate of voltage-translator transistor 90 is Vcc-0.4--0.4, or Vcc-0.8. The high input voltage to voltage-translator transistor 90 is reduced by the transistor threshold Vt, so the final output voltage is Vcc-0.8-Vt. When Vcc is 3.3 volts, and Vt is 0.8 volt, the high output voltage is 1.7 volt.
While such diode and threshold-based voltage translators are useful, the final output voltage cannot be an arbitrary value that can be optimized for specific interface requirements. Instead, the output voltage is a multiple of a fixed diode voltage drop and a threshold drop. When another output voltage is desired, such as 1.8 volt rather than 1.7 volt, it cannot easily be obtained. Some conversions, such as from 3.3 volt to 2.0 volt, are difficult to obtain with diodes. Using one diode and the threshold drop with a Vcc of 3.3 volt produces 2.1 volts, not 2.0 volts.
Although these differences in target voltage appear small, extra power can be drawn by the inexact voltages. These differences can actually be much larger as process and operating conditions change. Higher target voltages could damage the lower supply-voltage chips, but a lower target voltage compromises the system noise margin. More precise voltage translation is desirable.
The output voltage is fixed by the configuration of diodes and transistors in the voltage-translator chip. This prevents the user from adjusting the output voltage. Different voltage-translator chips must be used when the different target output voltages are needed. Multiple voltage-translator chips may need to be stocked, increasing costs. As power supply voltages continue to be reduced with succeeding generations of shrinking semiconductor technologies, different voltage-translator chips need to be used.
What is desired is a voltage translator using semiconductor transistors. It is desired to produce any arbitrary output voltage. It is further desired to reduce an input voltage to any desired output voltage using just one n-channel transistor with a carefully-controlled gate voltage. It is desired to eliminate diodes for voltage translation. It is further desired to have a user-programmable output voltage. A single voltage-translator chip is desired than can be used for different, user-programmable output voltages. It is also desired to track temperature, process, and power-supply variations.